Many industrial applications require adhesion to at least one plastic surface. Typical of such applications are paper and plastic (e.g. polypropylene) labeling of plastic (e.g. polyethylene terephthalate "PET") bottles, bonding of high density polyethylene basecups (HDPE) to PET bottles as well as general plastic to plastic bonding. While the specific formulations differ and each application poses a unique set of problems with respect to adhesion and to the stresses to which the bonds are subjected, some problems are common to all. In particular, the hot melt adhesive compositions commonly used are based primarily on ethylene vinyl acetate copolymers, styrene butadiene or styrene isoprene block copolymers, polyethylene and mixtures thereof. These polymers are characterized by relatively poor thermal stability, a problem which is more predominantly manifested in plastic objects which are to be recycled.
Recent pressures from governmental and environmental agencies have placed more emphasis on the need for many plastic constructions to be prepared from completely recyclable components. The recycling operation generally involves heating and melting the bottles, containers or other plastic structures to temperatures in excess of the melting point of each of the components therein. The resultant molten plastic mass is then used to form a new structure using conventional extrusion molding, etc. operations. The temperatures encountered in the recycling operations cause even the small or residual quantities of adhesive to char and/or discolor upon prolonged heating to above 175.degree. C. or for shorter durations to temperatures or above about 200.degree. C. which are required for blow molding of the reclaimed plastic thereby visibly fouling or darkening the reclaimed plastic and significantly limiting the possible end uses thereof.
It is the purpose of the present invention to provide a thermally stable hot melt adhesive composition which will not discolor or otherwise foul the reclaimed plastic (e.g. PET, HDPE, etc.) during the production or processing thereof. The use of the adhesives disclosed herein permits increased utilization of reclaimed plastics into markets demanding visually appealing materials. These markets include, but are not limited to, non-food grade PET containers and bottles, plastic trays and carpet fibers.